In order to find a hypothetical enzyme (cytochrome c synthetase) which would catalyze the covalent bonding of heme to apocytochrome c, Saccharomyces cerevisiae is chosen as an enzyme source. Synthesis tests have been carried out using 125I labeled apocytochrome c and heme as substrates, and the yeast mitochondria as presumably enzyme enriched cellular fractions. The extent of labelled apoprotein incorporation into the fraction of cytochrome c added as the carrier has been as much as 7% (0.35 nmol) of the extracted radioactive material. This radioactive material, resembling cytochrome c, will be further investigated in order to establish the exact nature of the observed phenomenom.